Switching motor drivers connected to large motors with high parasitic capacitance generate substantial noise current and voltage in the cable to the motor. That noise is a well-known source of electromagnetic interference (EMI) to nearby equipment. Previous approaches attenuate, but do not eliminate, this problem by using one or more of the following techniques: installing ferrite beads on cables connecting driver to motor, applying a snubber filter with aggressive shielding, using a low capacitance motor, using linear amplifiers, and using rate controlled switching amplifiers.
The disadvantages of ferrite beads and snubber filters include an inability to mitigate EMI at a level required for aircraft and military applications. In addition, available filters and properly terminated cable shields only reduce or contain the EMI problem until corrosion on shield terminations occurs. Similarly, aggressive shielding poses a high EMI risk after several years due to corrosion. Low capacitance motors, which can feature air or epoxy cores in place of soft magnetic materials, have a much lower torque per mass and lower efficiency than motors having magnetic cores. Linear or rate controlled switching amplifiers consume much more power than alternative amplifiers, are relatively inefficient, require large heat sinks, and become impractical at higher current levels. Moreover, linear amplifiers have a significantly lower bandwidth, are only used in low speed applications, and are typically unsuitable for anything other than relatively small motors. Accordingly, it would be desirable to provide a motor driver capable of powering relatively high capacitance motors while exhibiting acceptably low levels of EMI radiation, and that did not require EMI mitigation measures that were unreliable or that required compromised motor or driver configurations.